Reflow furnace for an electronic assembly

ABSTRACT

The invention provides a reflow furnace for an electronic assembly. The electronic assembly comprises a printed circuit board and a device on the printed circuit board. The printed circuit board has solder at a first area near the device and a metallic surface at second area distant from the device. The furnace comprises a frame, a support, a heater, and a shield. The support is secured to the frame and is capable of holding the printed circuit board. The heater is secured to the frame and is capable of heating the printed circuit board while being held by the support. The shield is secured to the frame and is positioned to prevent solder from migrating from the first area to the metallic surface at the second area while the printed circuit board is being heated.

BACKGROUND OF THE INVENTION

1). Field of the Invention

This invention relates to a reflow furnace for an electronic assemblyand to a method of assembling an electronic assembly.

2). Discussion of Related Art

An electronic assembly such as a motherboard or a card of a computertypically includes a printed circuit board to which one or more devicessuch as semiconductor packages including processors, memory etc. aremounted. A device is located on the printed circuit board and solderpaste is applied to the printed circuit board and a conductive wire, orthe like, of the device. The electronic assembly is then heated to causereflow of the solder. The electronic assembly is then allowed to coolwhich causes solidification of the solder. The solder thereby provides amechanical and electrical connection between the device and the printedcircuit board.

The printed circuit board may have metallic contacts on one or moreedges thereof. When the electronic assembly is heated, the solder startsmigrating to the metallic contacts. The solder tends to travel across asurface of the printed circuit board. The printed circuit board may alsohave hollow vias therein. Tiny solder balls may be located within thevias which blow out of the vias when the electronic assembly is heated.Solder migrating to the metallic contacts can attach to the metalliccontacts and so render the electronic assembly defective.

SUMMARY OF THE INVENTION

The invention provides a reflow furnace for an electronic assembly. Theelectronic assembly comprises a printed circuit board and a device onthe printed circuit board. The printed circuit board has solder at afirst area near the device and a metallic surface at second area distantfrom the device. The furnace comprises a frame, a support, a heater, anda shield. The support is secured to the frame and is capable of holdingthe printed circuit board. The heater is secured to the frame and iscapable of heating the printed circuit board while being held by thesupport. The shield is secured to the frame and is positioned to preventsolder from migrating from the first area to the metallic surface at thesecond area while the printed circuit board is being heated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example with reference tothe accompanying drawings wherein:

FIG. 1 is a sectioned side view illustrating a conveyorized reflowfurnace according to an embodiment of the invention;

FIG. 2 is a sectioned end view on 2--2 in FIG. 1;

FIG. 3 is a sectioned end view on 3--3 in FIG. 1, further illustratingan electronic assembly;

FIG. 4 is a graph illustrating how the electronic assembly is heated bythe furnace;

FIG. 5 is a side view illustrating a portion of a shield of the furnaceand a printed circuit board of the electronic assembly, before theprinted circuit board contacts a leading edge of the shield;

FIG. 6 is a view similar to FIG. 5 after the printed circuit boardcontacts the leading edge;

FIG. 7 is a view similar to FIG. 2 further illustrating the electronicassembly;

FIG. 8 is a plan view illustrating a portion of the electronic assemblyand a portion of the shield.

DETAILED DESCRIPTION OF THE INVENTION

A Conveyorized reflow furnace for heating an electronic assemblycomprising a printed circuit board and a device thereon is described.The furnace includes a frame to which a conveyor and heaters aremounted. The conveyor conveys the electronic assembly past the heatersto cause reflow of solder on the printed circuit board. A shield is alsomounted to the frame. The shield is made of a compliant material capableof withstanding high temperatures. A contacting edge of the shieldcontacts the printed circuit board and prevents migration of the solderto metallized contacts in the form of gold contacts located on an edgeof the printed circuit board.

FIG. 1 and FIG. 2 illustrate a conveyorized reflow furnace 10 accordingto an embodiment of the invention. The furnace 10 includes a frame 12, aconveyor 14, a first shielding arrangement 18A, and a second shieldingarrangement 18B.

The frame 12 includes first and second vertically extending supportstructures 20A and 20B, and first and second oven rails 22A and 22B. Thesupport structures 20A and 20B are horizontally spaced from one another.The first oven rail 22A is mounted to the first support structure 20Aand the second oven rail 22B is mounted to the second support structure20B.

The conveyor 14 includes first and second chains 24A and 24B, and firstand second sets of pin covers 26A and 26B. Each chain 24 is in the formof a closed loop and runs over gears 28 which are rotatably mounted tothe support structure 20A and 20B. The chain 24 thus has an uppersection 30 and a lower section 32. When the gears 28 are rotated, theupper section 30 moves in a direction 34 and the lower section 32returns in a direction opposite to the direction 34. The pin covers 26Aare mounted along the length of the first chain 24A and the pin covers26B are mounted along a length of the second chain 24B. The pin covers26A and 26B thus move together with the chains 24A and 24B in an orbitalpath. Each pin cover 26 includes a base portion 36 which is secured tothe chain 24, and a pin 38 extending from the base portion 36. The pins38 are located between the base portions 36 of opposing ones of the pincovers 26A and 26B.

The heaters 16 are located above the first section 30 of the chain 24and are mounted to the frame 12.

Each shielding arrangement 18 includes a respective bracket 42 securedto a respective one of the oven rails 22, and a respective shield 44secured to the bracket 42.

The bracket 42 includes a shield plate 46 having a slot 48 formedtherein. A threaded mounting shaft 50 is secured to the oven rail 22 andextends upwardly therefrom. The shield plate 46 is located over theshaft 50 so that the shaft 50 extends through the slot 48. A nut 52 islocated on the shaft 50 and rotated until a lower surface of the nut 52contacts an upper surface of the shielding plate 46. The shielding plate46 can then be adjusted because the slot 48 allows for movement of theshield plate 46 towards and away from the other shielding arrangement.The nut 52 can be tightened to secure the shielding plate 46 inposition.

The shield 44 is made of a compliant material such as a silicone rubbermaterial, and should preferably be able to withstand a temperature of atleast 230° C. without substantial, more preferably without any permanentdeformation, even under conditions wherein the shield 44 is bent. Theshield 44 is preferably made of a silicone rubber material as sold byMcMaster-Carr of Atlanta, Ga., capable of withstanding a temperature of260° C. and being about 0.79 millimeters thick. The shield plates 46have inner surfaces that slant inwardly towards one another in adownwardly direction. Each shield 44 is located against a respective oneof the slanted surfaces. A mounting plate 52 is located against theshield 44. A bolt 54 is inserted through a hole in the mounting plate 52and a slot in the shield 44, is threadably engaged with a threaded hole56 extending into the shield plate 46, and tightened. The shields 46 areso positioned so that lower portions thereof extend downwardly andinwardly from the brackets 42, each portion having a lower contactingedge 58. The contacting edge 58 is located lower than an upper surfaceof the pin 38. A slot 60 is formed in an upper portion of the shield 44which allows for adjustment of the shield 44 in an inclined plane inwhich the shield 44 is located.

FIG. 3 is a cross-section on 3--3 in FIG. 1. It shows that, in use, theelectronic assembly 62 is located on the conveyor 14 at an entrance ofthe furnace 10. The electronic assembly 62 includes a printed circuitboard 64 and one or more electronic devices 68. The printed circuitboard 64 has first and second opposed edges 70. The electronic devices68 may be semiconductor packages including processors, memory or otherdice and are mounted in a central area on top of the printed circuitboard 64. Solder (not shown) is typically located on the printed circuitboard 64 around the electronic devices for purposes of later connectingthe electronic devices 68 electrically and mechanically to the printedcircuit board 64. The printed circuit board 64 also typically includesvias (not shown) which are hollow. It may happen from time to time thattiny solder balls are trapped within the vias. Metallic surfaces such asgold contacts may be formed on one or both of the edges 70 of theprinted circuit board 64. It is necessary to keep the solder on thecentral region of the printed circuit board from reaching the contactson the edges 70 of the printed circuit board 64 when the electronicassembly 62 is heated.

The printed circuit board 64 is positioned so that one of the edges 70is located on the pins 38 of two or more of the first pin covers 26A andthe opposing edge 70 is located on the pins 38 of two or more of thesecond pin covers 26B. The edges 70 fit snuggly between inner surfacesof the base members 36.

The electronic assembly 62 is then conveyed past the heaters 16. Asshown in FIG. 4, the heaters 16 are set to gradually increase thetemperature of the electronic assembly 62 to about 160° over a period ofabout 30 seconds. The electronic assembly 62 is then further conveyedduring which time the temperature thereof remains substantially constantfor another 210 seconds. The electronic assembly is then graduallyheated to a temperature of about 230° C. The solder on the printedcircuit board reflows at a temperature of about 183° C. so as to beginsoftening or melting. The softened solder starts migrating by travelacross a surface of the printed circuit board 64 and by blowing out ofvias of the printed circuit board 64. The shielding arrangements 18A and18B are located in a region of the furnace 10 where the temperature isincreased above 160° C. to prevent migration of solder to the probecontacts on the edges 70 of the printed circuit board 64.

As shown in FIG. 5, each shield 44 has a curved leading edge 74. Becauseof the curvature of the edge 74, the printed circuit board 64 engageswith the edge 74 which is at angle other than 90° relative to adirection of movement of the printed circuit board 64. Due to the angleof contact of the printed circuit board 64 with the edge 74, and theinclination of the shield 44, the contacting edge 58 of the shield 44 islifted as shown in FIG. 6. The contacting edge 58 then lies on top ofthe printed circuit board 64.

FIG. 7 illustrates the electronic assembly 62 when the printed circuitboard 64 is moved in underneath the shields 44. The contacting edge 58of each shield 44 is bent by the printed circuit board 64 in a directionupwardly and away from the bracket 42 to which the respective shield 44is mounted. The exact location of the contacting edge 58 can be adjustedby adjusting the bracket 42 relative to the oven rail 22 in a directionparallel to a plane of the printed circuit board 64. The width ofcontact of the shield 44 with the printed circuit board 64 can also beadjusted by adjusting the shield 44 relative to the bracket 42 in aplane of the shield 44 which is at an angle and transverse to the planeof the printed circuit board 64.

FIG. 8 illustrates portions of the electronic assembly 62 and one of theshields 44. The contacting edge 58 of the shield 44 is located betweenthe device 68 and a plurality of contacts 80 on the edge 70 of theprinted circuit board 64. The shield 44 shields migrating of solder froman area around the device 68 to the contacts 80. The integrity of thecontacts 80 is thus maintained.

Referring again to FIG. 4, the electronic assembly 62 is then furtherconveyed to a cooler section of the furnace 10 and the temperature dropsto below 183° C. so as to cause solidification of the solder andmechanical attachment of the devices 68 to the printed circuit board 67.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it s to be understood that such embodiments aremerely illustrative and not restrictive of the current invention, andthat this invention is not restricted to the specific constructions andarrangements shown and described since modifications may occur to thoseordinarily skilled in the art.

What is claimed:
 1. A reflow furnace for an electronic assemblycomprising a printed circuit board and a device on the printed circuitboard, the printed circuit board having solder at a first area near thedevice and a metallic surface at a second area distant from the device,the furnace comprising:a frame; a support, secured to the frame, capableof holding the printed circuit board; a heater, secured to the frame,capable of heating the printed circuit board while being held by thesupport; and a shield secured to the frame, positioned to prevent solderfrom migrating from the first area to the metallic surface at the secondarea while the printed circuit board is being heated.
 2. A reflowfurnace according to claim 1, wherein the shield has a contacting edgepositioned to contact the printed circuit board.
 3. A reflow furnaceaccording to claim 2, wherein the shield is made of a compliant materialwhich allows the contacting edge to be bent by the printed circuitboard.
 4. A reflow furnace according to claim 3, wherein the materialcan withstand a temperature of at least 230° C. without substantialpermanent deformation of the shield.
 5. A reflow furnace according toclaim 1, wherein the positioning of the shield is adjustable relative tothe frame so that an edge of the shield is adjustable towards or awayfrom the device.
 6. A reflow furnace according to claim 5, furthercomprising a bracket, secured to the frame, the shield being secured tothe bracket and being adjustable relative to the bracket to adjust thepositioning of the shield.
 7. A reflow furnace according to claim 6,wherein the bracket is adjustable relative to the frame to adjust thepositioning of the shield.
 8. A reflow furnace according to claim 3,wherein the positioning of the shield is adjustable relative to theframe so that a contacting edge of the shield is adjustable towards oraway from the device.
 9. A reflow furnace according to claim 8, furthercomprising a bracket, secured to the frame, the shield being secured tothe bracket and being adjustable relative to the bracket to adjust thepositioning of the shield.
 10. A reflow furnace according to claim 9,wherein the bracket is adjustable relative to the frame to adjust thepositioning of the shield.
 11. A reflow furnace according to claim 10,wherein the bracket is adjustable towards and away from the device andthe shield is adjustable relative to the bracket towards and away fromthe printed circuit board in a direction transverse to a plane of theprinted circuit board.
 12. A reflow furnace according to claim 1,wherein the support includes:a member mounted to the frame for movementin an orbital path, the member being capable of supporting the printedcircuit board and movement of the member causing conveyance of theelectronic assembly.
 13. A reflow furnace according to claim 12, whereinthe shield has a contacting edge positioned to contact the printedcircuit board, the shield being made of a material which allows thecontacting edge to be bent by the printed circuit board.
 14. A reflowfurnace according to claim 13, wherein the shield has a leading edgewhich engages with the printed circuit board when the electronicassembly is conveyed towards the shield, the leading edge being at anangle other than 90° relative to a direction of movement of theelectronic assembly.
 15. A reflow furnace according to claim 14, whereinthe shield is in a plane which is at an angle other than 90° relative toa plane of the printed circuit board before the printed circuit boardengages therewith.
 16. A reflow furnace according to claim 13, whereinthe material can withstand a temperature of at least 230° C. withoutsubstantial permanent deformation of the shield.